1. Field of the Invention
The present invention relates to a backlight, and more particularly, to an apparatus and method for driving a backlight.
2. Description of the Related Art
Flat panel displays include plasma display panels, field emission displays, light emitting diodes, and liquid crystal display devices. Such flat panel displays are generally classified into emissive type flat panel displays and non-emissive type flat panel displays. Plasma display panels, field emission displays, and light emitting diodes are examples of emissive type flat panel displays.
Liquid crystal display devices are examples of non-emissive type flat panel displays. A liquid crystal display (LCD) is not self-luminous because it is a non-emissive type flat panel display. Thus, the LCD requires an external light source for displaying images.
Hence, a backlight is provided on a rear side of the LCD. Therefore, the LCD device can display images even in a dark environment by using light transmitted from the backlight assembly. The backlight assembly is required to provide high and uniform brightness, is highly efficient, has a long life, is thin and light, and costs less. For example, notebook computer monitors require a high-efficiency, long-life backlight assembly to reduce power consumption, and computer monitors and TV displays require a high-brightness backlight assembly.
The backlight assembly uses a lamp as a light source. Depending on the location of the lamp, the backlight assembly is classified as an edge type backlight assembly or a direct type backlight assembly. In the edge type backlight assembly, the lamp is disposed at a side of the LCD device and projects light in a horizontal direction. Then, a light guide plate directs the projected light to a liquid crystal panel at the front of the LCD device. In contrast, in the direct type backlight assembly, several lamps are arranged at uniform intervals. The lamps project light directly onto the liquid crystal panel at the front of the LCD device.
As discussed above, the backlight assembly must provide a high-brightness characteristic. As an example, the backlight assembly may use a cold cathode fluorescent lamp (CCFL) to achieve high-brightness. The backlight assembly incorporating the CCFL uses a step-up transformer to convert a low AC voltage of several tens of kilohertz (KHz) obtained from an inductive-capacitive (LC) resonant inverter into a high voltage required for discharging the CCFL. Here, the LC resonant inverter outputs a sinusoidal waveform. Although the LC resonant inverter is a relatively simple and highly efficient device, a single inverter can hardly drive several CCFLs in parallel. Hence, when using CCFLs, the edge type or direct type backlight assembly requires as many inverters as the number of CCFLs.
To avoid the drawbacks of the CCFL and provide high-brightness and high-efficiency characteristics for larger LCD devices, and long-lifespan and light-weight characteristics, an external electrode fluorescent lamp (EEFL) has been developed for the backlight assembly by forming an external electrode on an electrodeless glass tube.
The direct type backlight assembly can be constructed using a plurality of EEFLs by arranging the EEFLs on a reflection plate. In this case, a high brightness of several tens of thousands candelas per square meter (cd/m2) can be attained by driving the EEFLs at several megahertz (MHz). A single inverter can drive several such EEFLs connected in parallel.
If a plurality of EEFLs is driven in this way, a significantly large current may flow out of an output terminal of the inverter, that is, an output terminal of a transformer. Since this large current could be fatal to the human body, a protective limiting current circuit (LCC) is provided in the backlight assembly to turn off the EEFLs and prevent electrocution of a user.
FIG. 1 is a block diagram showing a backlight driving device provided with an EEFL according to the related art, and FIG. 2 is a graph showing voltage waveforms through the backlight driving device of FIG. 1. Referring to FIGS. 1 and 2, a controller 11 outputs a pulse width modulation (PWM) controller output signal to an FET 13 to drive a lamp 17 according to a PWM method. A DC voltage Vin (DC) is externally applied to the FET 13. In the FET 13, four transistors (not shown) are connected in parallel with one capacitor (not shown) in-between. The FET 13 outputs a FET output signal. A first pulse of the FET output signal produces a positive DC square voltage, and a second pulse of the FET output signal produces a negative DC square voltage. Thus, the FET 13 alternately outputs a positive DC square voltage and a negative DC square voltage in response to the continuously inputted pulses of the controller output signal.
The transformer 15 boosts each DC square voltage to a predetermined level and outputs the boosted voltages to the lamp 17. The lamp 17 includes a plurality of EEFLs connected in parallel with respect to the transformer 15.
Meanwhile, electrical characteristics of the transformer 15 or the lamp 17, such as voltage and current, are detected at an output side of the transformer 15. For instance, the voltage and current characteristics are measured between the transformer and the lamp 17. The detected electrical characteristics are supplied to a protective LCC 19.
If a human body comes in contact with an output of the transformer 15, the protective LCC 19 supplies an enable signal to the controller 11 to turn off the lamp 17. If there is no contact by a human body, the protective LCC 19 does not supply any signal to the controller 11, such that the backlight can be normally operated. For this, the protective LCC 19 determines whether the enable signal is to be supplied depending on the detected electrical characteristics.
A voltage of about 1500 V is provided to drive the lamp 17. However, the voltage decreases to a value equal to or lower than about 200 V if there is a contact with a human body having a resistance of about 2 kΩ. Thus, the protective LCC 19 supplies the enable signal to the controller 11 only when the voltage of the second side of the transformer 15 is about 200 V, and, otherwise, supplies no signal to the controller 11.
FIG. 3 is a graph of a current-voltage characteristic of the backlight driving device of FIG. 1. Referring to FIG. 3, when the transformer 15 is initially operated, it takes a rising time period for the output voltage of the transformer 15 to increase from 0 V to 1500 V. While increasing, the output voltage of the transformer 15 inevitably passes through a value of 200 V, which corresponds to the output voltage when a human body is contacted.
Thus, when the output voltage of the transformer 15 increases from 0 V to 200 V during the initial rising time, the protective LCC 19 determines that a human body is contacted and supplies an enable signal to the controller 11 to turn off the lamp 17, although there is no actual contact with a human body. Accordingly, the backlight assembly stops operating. Specifically, the protective LCC 19 improperly turns off the lamp 17 in response to the transformer output voltage reaching 200 V during the initial operation of the backlight assembly.
Thus, during startup, the related art backlight assembly functions improperly when it reaches a voltage of about 200 V corresponding to the human body contact voltage. Further, even if the related art backlight assembly is manually reset after the improper shutdown of the lamp during startup, the lamp keeps shutting down every time the backlight assembly is operated. Thus, proper operation of the backlight assembly is prevented.